GB2472575A

GB2472575A - Optical well monitoring system

Info

Publication number: GB2472575A
Application number: GB0913867A
Authority: GB
Inventors: Daniel Andrew Watley; Jackson Teck Leong Yeo
Original assignee: Sensornet Ltd
Current assignee: Sensornet Ltd
Priority date: 2009-08-10
Filing date: 2009-08-10
Publication date: 2011-02-16
Also published as: GB0913867D0

Abstract

A well monitoring system includes an optical source 27 connected to an input fiber 29. A detector system 205 is connected to an output fiber 202 and there is at least one input optical coupler 28 located along the input fiber 29, each input optical coupler 28 being connected to couple a portion of light in the input fiber 29 to an optical sensor 23. An output optical coupler 203 corresponding to each sensor 23 is connected to couple light from that sensor 23 to the output fiber 202. The arrangement is said to enable to control of a number of sensors with a single fiber, particularly in multilateral wells. Also claimed is a well monitoring system comprising, a pulsed optical source, an optical detector and a sensor, where the detector and sensor are configured to perform optical time domain reflectometry.

Description

Well Monitoring System

Background

This invention relates to monitoring conditions in a well, and in particular to the monitoring of conditions at a number of locations in the well, using optical techniques.

In the oil and gas exploration and production industry, wellbores are drilled into the earth to intercept subterranean hydrocarbon bearing formations or reservoirs to permit the hydrocarbons to be produced to surface. The conditions within the wellbore and formation can have a significant influence on many aspects associated with the well, such as well infrastructure, production rates and the like. Accordingly, it is extremely desirable to perform a number of measurements associated with the wellbore, and even the formation, to identify particular in situ conditions. Such measurements may include pressure, temperature, vibration, chemical composition, geological conditions or the like.

Optical techniques provide a convenient method for monitoring certain conditions in an oil well, for example the composition of the produced fluids. The fluid extracted from a well generally contains a mixture of substances, including hydrocarbons and water. The fluid may be in liquid or gas form, or a combination of both. In particular the relative proportion of oil and water is an important parameter to ensure optimum well operation. As explained in "Petroleum Development Oman's Experience in Using Near-lnfra Red Absorption Based Water Cut Meters for Well Testing and Production Monitoring, Khalil Al-Hanashi, Salim AlSibani, and John Lievois; SPE Middle East Oil and Gas Show and Conference, 15-18 March 2009, Bahrain, Bahrain" spectroscopy techniques can be utilised to determine the composition of a liquid extracted from an oil well. A probe is inserted into the fluid flow to pass light through a section of liquid flowing in a pipe to allow measurement of the absorption spectrum, and hence determination of the composition. This technique is, however, limited as it monitors the composition of the fluid arriving at the surface.

In multilateral wells fluids are extracted from a number of zones, but are mixed before their arrival at the surface since the fluids are extracted through a single surface outlet. The monitoring technique descried above can only therefore measure the final composition, and not that extracted from each zone. It would be desirable to monitor the fluid composition extracted from each of the zones so that a complete understanding of the well operation can be obtained. For example, if a particular zone is seen to have a high proportion of water, steps may be taken to adjust the water injection into that zone, or discontinue extraction from that zone. US Patent 6,507,401 discloses a downhole device for monitoring the composition of liquid in the well. An optical sensor is located in a short section of production tubing to pass light through fluid in the tubing. The sensor is connected to a light source and a detector located on the surface by optical fibres. Analysis of the light received at the receiver allows the composition of the fluid to be determined by inspection of the amount of light being received at one or more different wavelengths. Multiple sensors may be utilised to monitor composition at a number of locations, but each sensor requires two fibre optic connections to the surface, which incurs significant cost and may make the provision of multiple sensors impractical. Furthermore the technique for multilateral wells ideally requires a sensor located in each lateral well, but this is not practical in since there is often no tubing connection between the lateral wells and the motherbore.

A further disadvantage of the system of US 6,407,401 is that there is no provision for calibration of fibre or sensor loss changes over the life of the system. A particular problem with optical downhole sensors is hydrogen absorption into the fibres which significantly affects the loss of the fibre. This spectral loss changes over the life of the fibre as hydrogen is absorbed into the fibre and also changes between different wavelengths. Since the system measures the total loss between transmitter and receiver at one or more different wavelengths the changing fibre loss due to hydrogen absorption will affect accuracy of the measurement made.

There is therefore a need for a monitoring system to allow monitoring at a number of locations, and to address varying loss in the monitoring system.

Summary

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

There is provided an well monitoring system, comprising an optical source connected to an input fibre and configured to emit pulses of light into the input fibre, an detector system connected to an output fibre, at least one input optical coupler located along the input fibre, each input optical coupler being connected to couple a portion of light in the input fibre to an optical sensor, and an output optical coupler corresponding to each sensor, connected to couple light from that sensor to the output fibre.

There may be a plurality of optical input couplers, sensors and output optical couplers. The system according may further comprise an optical sensor connected between the input fibre and the output fibre.

The optical source may emit a single wavelength.

The optical source may emit a plurality of discrete optical wavelengths simultaneously.

The optical source may emit a plurality of discrete optical wavelengths sequentially.

The optical source may emit a broadband optical signal.

Each optical sensor may transmit light through a fluid in the production piping, and the detector may be configured to calculate an optical loss between the light source and the detector for each sensor.

Each optical sensor may transmit light through a fluid in the production piping, and the detector may be configured to calculate an optical loss at multiple wavelengths between the light source and the detector for each sensor.

The well monitoring system may further comprise an optical isolator at the input and/or output of at least one of the optical sensors.

The calculated loss may be is adjusted for the loss of the optical fibre in the system.

The sensor may comprise a mirror to reflect light from an input to the fluid to an output from the fluid.

There is also provided a method of monitoring a well utilizing a monitoring system as described above comprising the steps of transmitting an optical pulse into the input fibre, detecting an optical signal from the return fibre, and calculating a loss between the optical source and detector for each sensor.

The method may further comprise the step of determining a composition of a fluid at the sensors based on the calculated loss.

The duration of the optical pulse may be less than the smallest time difference between the round trip times through each sensor.

The method may further comprise the step of measuring the loss of the input or return fibre utilizing an OTDR, and adjusting the calculated loss according to that measurement. There is also provided a well monitoring system, comprising a pulsedoptical source and an optical detector system coupled to the proximal end of an optical fibre, at least one optical sensor connected to the optical fibre, wherein the optical source and detector are configured to perform optical time domain reflectometry measurements on the optical fibre and optical sensor.

The well monitoring system may comprise a plurality of optical sensors, wherein the plurality of sensors are connected serially along the length of the optical fibre.

The well monitoring system may be configured to detect the loss across at least one of the optical sensors.

The optical source may emit a single wavelength.

The optical source may emit a broadband optical signal.

There is also provided a method of monitoring a well utilizing a monitoring system as described immediately above comprising the steps of transmitting an optical pulse into the optical fibre, detecting a reflected optical signal from optical fibre, and calculating a loss of each sensor.

The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention.

Brief Description of the Drawings Embodiments of the invention will be described, by way of example, with reference to the following drawings, in which:

Figure 1 shows a schematic diagram of an optical sensor;

Figure 2 shows a schematic diagram of a well equipped with a multiple4ocation optical monitoring system;

Figure 3 shows an exemplary chart of signals in the system of Figure 2 during operation;

Figure 4 shows a schematic diagram of a well equipped with a multiple4ocation optical monitoring system connected using a single fibre;

Figure 5 shows an exemplary chart of signals in the system of Figure 4 during operation; and

Figure 6 shows an alternative form of the optical sensor of Figure 1.

Detailed Description

Embodiments of the present invention are described below by way of example only. These examples represent the best ways of putting the invention into practice that are currently known to the Applicant although they are not the only ways in which this could be achieved. The description sets forth the functions of the example and the sequence of steps for constructing and operating the example. However, the same or equivalent functions and sequences may be accomplished by different examples.

Figure 1 shows a cross-section of a well pipe 10 including an optical sensor for monitoring liquid flowing in the pipe. A recess 11 is located in the pipe wall with transparent windows 12, 13 located at either side to allow light to be transmitted through the fluid. Optical fibres 14, 15 are configured to inject light through one of the windows 12 and collect light transmitted out through the other window 13. The fibres 14, 15 are connected to a light source and a receiver respectively.

Figure 2 shows a schematic diagram of a well equipped with a multiple4ocation optical monitoring system. The main well 20 has two lateral wells 21 , 22 from which oil is extracted. Three sensors 23, 24, 25 as shown in Figure 1 are provided to allow monitoring of the fluids extracted from the main well 20 and each of the lateral wells 21 , 22. The location of the sensors 23, 24, 25 allows the fluid flowing out of each of the wells 20, 21 , 22 to be analysed independently.

The fluid from the main well 20 is measured directly by the third sensor 25. The composition of the liquid from the second lateral well 22 is calculated by subtracting the composition of the liquid at the third sensor 25 from the measured composition at the second sensor 24 (since the liquid at the second sensor is a mixture of the liquid from the main well 20 and the second lateral well 22. A comparable technique is utilised to calculate the composition of the liquid from the first lateral well 21.

An input fibre 26 is connected between a light source 27 and a first optical coupler 28, which splits light between a fibre connected to the first sensor 23, and a second input fibre 29. The second input fibre 29 is connected to a second optical coupler 200 which splits light between a fibre connected to the second sensor 24, and a third input fibre 201. The third input fibre is connected to the third sensor 25. This fibre and coupler structure distributes a single light source to all three couplers. The ratios of the couplers may be selected to deliver an equal power of light to each sensor, or any defined split as required by the loss and sensing budget of the system.

A comparable connection is provided by a return fibre 202 and couplers 203, 204 to couple light from the output of the sensors to a detector system 205.

The system allows multiple sensors to be connected in parallel, using only two main fibre optic cables. Increasing the number of sensors does not therefore lead to a significant increase in the fibre required (each additional sensor requires two couplers, the short length of fibre from the coupler to the sensor, and any additional input and return fibre if the sensor is more distant from the surface of the well).

Each sensor of Figure 2 can be analysed independently by using a pulsed light source, since the transmission time through each of the sensors is different due to the different fibre lengths. A pulse of light launched from light source 27 will therefore lead to three pulses being received at the detector system 207, as shown in Figure 3.

The dashed line 30 indicates the transmission of a pulse of light from the light source 27. The light coupled through the first sensor 23 follows the shortest path and so returns to the analyser as pulse 31. A second pulse 32 is received from the second sensor 24 after the roundtrip time through that sensor, and similarly a third pulse 33 is received from the third sensor 25. The returned pulses decrease in power for each sensor as the light travels through more fibre and hence suffers greater attenuation.

The pulse width launched into the fibre must be sufficiently small that the pulses returning to the detector system do not overlap, and thus the maximum length is defined by the smallest difference in round-trip time between two sensors. The light source may be a broadband source, or may be a plurality of sources at discrete wavelengths located at relevant locations of the spectrum. In the case of multiple sources, the wavelengths may be transmitted at discrete times, or two or more of the wavelengths may be transmitted at the same time. A suitable detector and analysis system is provided at the detector system as is known in the art for detecting pulsed signals at varying wavelengths.

The wavelengths utilised may be as described in US 6,507,401 , the above-reference paper, or may be different wavelengths selected for the particular liquids present in the well concerned.

The received signals may be analysed as described in US 6,507,401 or using other known techniques to determine the compositional information required.

Figure 4 shows a further monitoring system for monitoring multiple locations in a well. As described in relation to Figure 3 three sensors 23, 24, 25 of the type shown in Figure 1 are located in the well. In contrast to Figure 2, the sensors are connected serially using a single fibre 40. At the surface end of the fibre a coupler 41 allows the injection of a pulsed optical signal into the well fibre from an optical source 42, and the detection of a returning signal by a detector system 43. The distal, down-hole, end of the fibre is terminated either in an antireflection absorber, or by a reflector to provide a known reference point for calibration purposes.

Measurement is performed using a reflective time domain technique. A pulse of light is launched into the fibre and the detector detects reflected light as a function of time. The power of light received at each point in time is utilised to calculate the loss to each point along the fibre. This technique is commonly referred to as Optical Time Domain Reflectometry (OTDR), and the skilled person will understand how to implement a measurement system using this technique in the system of Figure 4.

As described previously, the light source may be a broadband wavelength source, or a plurality of discrete wavelengths for transmission together, or at discrete times.

Figure 5 shows an example signal detected by the detector system 43. In the region 50 the loss is that of the well fibre to the first sensor 23. The discontinuity 51 relates to the first sensor 23 where a more substantial loss occurs. The loss in the distance region corresponding to the sensor is equal to the loss of the sensor and hence gives the absorption of the fluid. This measurement at a number of wavelengths allows analysis of the liquid composition as described hereinbefore. Similar loss measurements for the second and third sensors 24, 25 are made from steps 52, 53 in the loss graph. Independent analysis of each sensor is therefore possible. The system of Figure 4 measures the absorption at each sensor independently from the loss of the fibre connecting the sensor to the source and detector and is therefore not affected by the changes in the loss in the fibre due to hydrogen absorption. Such a system is not restricted to more than one sensor, but may also find application with a single sensor due to the ability to compensate for varying fibre losses.

In Figure 4 the fibre is terminated after the final sensor, but in a modified system the fibre may be continued to return to the surface to form a loop of fibre. In such a system light pulses can then be injected into either end of fibre, allowing interrogation of the loop in both directions. This reduces the losses to the sensors which are most distal in the first direction, thereby improving the optical power at those sensors, which may allow an improvement in accuracy. Measurement of the system in both directions may provide an improvement in overall system accuracy.

Figure 6 shows a variation of the sensor of Figure 1. The sensor has a transparent window 60 that acts as an input and output for the sensor. An input fibre 61 injects lights through the window 60 into the recess 62. The light passes through the liquid in the recess and is reflected by a mirror 63. The light then passes back through the liquid and out of the window 60. An output fibre 64 collects the returning light. A lens or other arrangement may be utilised to couple light between the fibres and the sensor.

The sensor operates in the same manner as that of Figure 1 , and may be connected into systems in the same way as that sensor, as has been described hereinbefore. The use of a single input output window and a mirror allows an increase in the path length through the liquid (since the light travels across the recess twice) and may allow an improvement in measurement accuracy).

In a modification of the system of Figure 2 an optical isolator may be provided on the input and/or output of each of the sensors, to allow the use of an OTDR device to determine the loss of the fibres in the system, thus allowing varying fibre loss to be calibrated for. In the system shown in Figure 2 an OTDR cannot be utilised (apart from for the length of fibre to the first optical coupler) because there are multiple optical paths having the same length. The signals from each path will overlie, thereby preventing a measurement of the fibre loss for a single path. An optical isolator at the input and output of each sensor removes any multiple paths of the same length (since it prevents the OTDR signal returning to the input/output fibre from the parallel path through the sensor) thereby allowing an OTDR measurement to be made of the loss of the input and return fibres. An isolator may be provided on only the input or the output of each sensor. This does lead to a short section where there are multiple paths (where the sensor overlies the input/return fibre), but the sensor lengths are short compared to the input/return fibres and so the regions in which accurate measurements cannot be made are small.

The phrase "oil well" has been used as an example only, but as will be appreciated the techniques and apparatus disclosed herein are independent of the particular type of well and may find application in any type or configuration of well. Similarly, references to oil or particular fluids are for example only and the techniques can be readily adapted to measuring any composition by selection of appropriate wavelengths for the spectral properties of the liquid being measured.

Three sensors have been shown in the figures by way of example only, but as will be appreciated the techniques described herein are applicable to any number of sensors. The number of sensors may be limited by the optical budget of the system.

The above description has been given in relation to sensors utilising the absorption of liquids to determine the composition of liquid, but as will be appreciated the techniques are also applicable to other optical sensors for monitoring a range of physical properties in the well. For example, the sensors shown in Figures 1 and 5 may be utilised in a fluorescence measurement, or different types of sensor such as optical interferometers may be utilised to measure other parameters such as pressure.

The application of the disclosed measurement system to multilateral wells has been described, but as will be appreciated the techniques and apparatus are also applicable to any type of well. For example, monitoring at multiple locations may be beneficial for single bore wells which produce fluids at multiple locations along the length of the bore to allow analysis of the fluids obtained from each zone along the well.

The term optical tap may be utilised to describe the optical couplers mentioned hereinbefore to reflect that their function is to tap-off a portion of the power in the input fibre, or couple power into the output fibre, where the input fibre continues beyond the tap.

As will be appreciated the wells referred to herein may by used to extract a variety of fluids in both the liquid and gas states, or mixtures of both. The word fluid is therefore used herein to refer to any liquid, gas or mixture extracted from a well.

It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. Any reference to 'an' item refers to one or more of those items. The term 'comprising' is used herein to mean including the method blocks or elements identified, but that such blocks or elements do not comprise and exclusive list and a method or apparatus may contain additional blocks or elements.

The steps of the methods described herein may be carried out in any suitable order, or simultaneously where appropriate.

It will be understood that the above description of a preferred embodiment is given by way of example only and that various modifications may be made by those skilled in the art. Although various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention.

Claims

Claims 1. An well monitoring system, comprising an optical source connected to an input fibre and configured to emit pulses of light into the input fibre, an detector system connected to an output fibre, at least one input optical coupler located along the input fibre, each input optical coupler being connected to couple a portion of light in the input fibre to an optical sensor, and an output optical coupler corresponding to each sensor, connected to couple light from that sensor to the output fibre.
2. A well monitoring system according to claim 1 , comprising a plurality of optical input couplers, sensors and output optical couplers.
3. A well monitoring system according to claim 1 or claim 2, further comprising an optical sensor connected between the input fibre and the output fibre.
4. A well monitoring system according to any preceding claim wherein the optical source emits a single wavelength.
5. A well monitoring system according to any of claims 1 to 3 wherein the optical source emits a plurality of discrete optical wavelengths simultaneously.
6. A well monitoring system according to any of claims 1 to 3 wherein the optical source emits a plurality of discrete optical wavelengths sequentially.
7. A well monitoring system according to any of claims 1 to 3 wherein the optical source emits a broadband optical signal.
8. A well monitoring system according to any preceding claim, wherein each optical sensor transmits light through a fluid in the production piping, and the detector is configured to calculate an optical loss between the light source and the detector for each sensor.
9. A well monitoring system according to any of claims 5 to 7, wherein each optical sensor transmits light through a fluid in the production piping, and the detector is configured to calculate an optical loss at multiple wavelengths between the light source and the detector for each sensor.
10. A well monitoring system according to any preceding claim, further comprising an optical isolator at the input and/or output of at least one of the optical sensors.
11. A well monitoring system according to claim 8 or claim 9 wherein the calculated loss is adjusted for the loss of the optical fibre in the system.
12. A well monitoring system according to claim 8 or claim 9 wherein the sensor comprises a mirror to reflect light from an input to the fluid to an output from the fluid.
13. A method of monitoring a well utilizing a monitoring system as claimed in any of claims 1 to 12 comprising the steps of transmitting an optical pulse into the input fibre, detecting an optical signal from the return fibre, and calculating a loss between the optical source and detector for each sensor.
14. A method of monitoring a well as claimed in claim 13 further comprising the step of determining a composition of a fluid at the sensors based on the calculated loss.
15. A method of monitoring a well as claimed in claim 13 or claim 14, wherein the duration of the optical pulse is less than the smallest time difference between the round trip times through each sensor.
16. A method of monitoring a well as claimed in any of claims 13 to 15, further comprising the step of measuring the loss of the input or return fibre utilizing an OTDR, and adjusting the calculated loss according to that measurement.
17. A well monitoring system, comprising a pulsed optical source and an optical detector system coupled to the proximal end of an optical fibre, at least one optical sensor connected to the optical fibre, wherein the optical source and detector are configured to perform optical time domain reflectometry measurements on the optical fibre and optical sensor.
18. A well monitoring system according to claim 17, comprising a plurality of optical sensors, wherein the plurality of sensors are connected serially along the length of the optical fibre.
19. A well monitoring system according to claim 17 or claim 18 wherein the detector system is configured to detect the loss across at least one of the optical sensors.
20. A well monitoring system according to any of claims 17 to 19 wherein the optical source emits a single wavelength.
21. A well monitoring system according to any of claims 17 to 19 wherein the optical source emits a plurality of discrete optical wavelengths simultaneously.
22. A well monitoring system according to any of claims 17 to 19 wherein the optical source emits a plurality of discrete optical wavelengths sequentially.
23. A well monitoring system according to any of claims 17 to 19 wherein the optical source emits a broadband optical signal.
24. A well monitoring system according to any of claims 17 to 23, wherein each optical sensor transmits light through a fluid in the production piping, and the detector is configured to calculate an optical loss between the light source and the detector for each sensor.
25. A well monitoring system according to any of claims 17 to 23, wherein each optical sensor transmits light through a fluid in the production piping, and the detector is configured to calculate an optical loss at multiple wavelengths between the light source and the detector for each sensor.
26. A method of monitoring a well utilizing a monitoring system as claimed in any of claims 17 to 25 comprising the steps of transmitting an optical pulse into the optical fibre, detecting a reflected optical signal from optical fibre, and calculating a loss of each sensor.
27. A method of monitoring a well as claimed in claim 25 further comprising the step of determining a composition of a fluid at the sensors based on the calculated loss